Electrolytic machining method and electrolytic machining apparatus

ABSTRACT

An electrolytic machining method includes electrolytically machining a workpiece that is positioned opposite to an electrode tool while filling an electrolytic solution between the workpiece and the electrode tool and applying a current across the workpiece and the electrode tool. The electrolytic machining is performed by having at least a part of opposing sections of the workpiece and the electrode tool immersed in the electrolytic solution reserved in a machining and storing section, while the machining surface of the workpiece is positioned at a depth of about 5 mm to about 35 mm from the surface of the electrolytic solution reserved in the machining and storing section, and by supplying the electrolytic solution to be filled in a gap at the opposing sections between the workpiece and the electrode tool.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electrolytic machining method, in which an electrolytic machining is performed on a workpiece that is positioned opposite to an electrode tool by filling an electrolytic solution between the workpiece and the electrode tool and applying a current across the workpiece and the electrode tool, a method for fabricating grooves for dynamic pressure bearings, and dynamic pressure bearing devices manufactured according to the manufacturing method.

[0003] 2. Related Background Art

[0004] Electrolytic machining is performed by concentrating electrodissolution on certain parts of a workpiece as required, and an electrolytic machining apparatus indicated in FIG. 22, for example, has been known for sometime. In the electrolytic machining apparatus shown in FIG. 22, a workpiece 4 is placed on a jig 3, which is mounted on a base 1 via an insulating material 2, and an electrode tool 5 is placed opposite to the workpiece 4 in close proximity. The workpiece 4 is connected to the positive side (+pole) of a power supply for electrolytic machining, which is omitted from drawings, while the electrode tool 5 is connected to the negative side (−pole).

[0005] In the meantime, an electrolytic solution 6 collected externally is supplied by a pump 7, which is a device for supplying an electrolytic solution, through a filter 8 to a gap between the electrode tool 5 and the workpiece 4. While the electrolytic solution 6 is filled between the electrode tool 5 and the workpiece 4, a current is applied across the electrode tool 5 and the workpiece 4. This causes the workpiece 4 to electrochemically elute, such that the workpiece 4 is electrolytically machined.

[0006] A feeder 10 is installed on the electrode tool 5. The electrode tool 5 is fed by the feeder 10 into the workpiece 4 as the machining on the workpiece 4 progresses, and this allows a predetermined machining gap (equilibrium gap) between the two to be maintained, which consequently allows a shape that is an inversion of the shape of the electrode tool 5 to be formed on the workpiece 4. Gas that is generated by the electrolytic machining is ventilated outside by a fan 11. In addition, the electrolytic solution whose temperature rises due to the Joule heat contains various types of electrolytic products; a used electrolytic solution 12 is purified by a centrifuge 13 and subsequently supplied into the gap between the electrode tool 5 and the workpiece 4 again.

[0007] However, a mass production machining process using such a general electrolytic machining method entails the following problems:

[0008] (1) There is a tendency for machining widths of workpieces to be larger than the widths of electrode tools, and fluctuations in the machining widths tend to occur.

[0009] (2) When the gap between the electrode tool and the workpiece is narrowed in order to reduce the fluctuations in machining widths, various types of particles in the electrolytic solution such as electrolytic products from the workpieces tend to cause clogging, which often leads to defective machining.

[0010] (3) Similarly, when the gap between the electrode tool and the workpiece is made narrower, the flow of the electrolytic solution is less smooth, which causes deterioration of the electrolytic solution during machining; this results in deep machining depth near the supply side of the electrolytic solution and gradually shallower machining depth towards the drain side; and

[0011] (4) Some electrolytic solution and/or electrolytic products tend to adhere to the workpiece.

[0012] When electrolytic machining is used in groove machining of dynamic pressure generating grooves in a dynamic pressure bearing device that utilizes the dynamic pressure of a lubricating fluid, the shapes of the dynamic pressure generating grooves that have a great impact on the dynamic pressure property cannot be obtained at the precision required. This not only causes a failure to obtain favorable dynamic pressure property, but also leads to lower productivity. Furthermore, when electrolytic products or the electrolytic solution remain attached to a processed product, they become chemical debris on certain types of rotating bodies that are supported by dynamic pressure bearing devices, such as hard disk drive devices (HDD), and cause the rotating bodies to be inoperable.

SUMMARY OF THE INVENTION

[0013] In view of the above, the present invention relates to an electrolytic machining method, as well as a method for manufacturing dynamic pressure bearing grooves, in which workpieces can be processed efficiently and with high precision using a simple structure.

[0014] In order to achieve the objective, in an electrolytic machining method in accordance with an embodiment of the present invention, an electrolytic machining is performed by having at least a part in which a workpiece and an electrode tool oppose each other is immersed in an electrolytic solution retained in a machining and storing section, while the machining surface of the workpiece is positioned at a depth of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section, and by supplying the electrolytic solution to be filled in a gap between the workpiece and the electrode tool in the part where they oppose each other.

[0015] According to the electrolytic machining method having such a structure, due to the fact that the machining surface of the workpiece is positioned at a depth of about 5 mm or deeper from the surface of the electrolytic solution retained in the machining and storing section, there is virtually no amount of air, especially oxygen, entering the electrolytic solution, which ensures a high quality electrolytic machining. At the same time, due to the fact that the machining surface of the workpiece is positioned at a depth of about 35 mm or less from the surface of the electrolytic solution retained in the machining and storing section, the fluidity of the electrolytic solution is maintained favorably, which allows a smooth elimination of electrolytic products after the electrolytic machining.

[0016] The material of a shaft member or a bearing member used in a dynamic pressure bearing device that utilizes the dynamic pressure of a lubricating fluid may be used as the workpiece described above. Due to the fact that dynamic pressure generating grooves are formed as concave sections in the workpiece, the dynamic pressure generating grooves are machined with especially high precision.

[0017] In one aspect of the present embodiment, a masking member that has continuous hole patterns formed as through-holes that correspond to the shapes of concave sections may be adhered to the machining surface of the workpiece described above. As a result, the electrolytic solution is supplied to flow in a gap between the masking member and the electrode tool in order to allow the electrolytic solution to enter into the continuous hole patterns of the masking member and thereby allow an electrolytic machining to take place, and thus the electrolytic solution supplied to the workpiece flows only within the continuous hole patterns of the masking member that is adhered to the workpiece. Consequently, even when the fluidity of the electrolytic solution is improved by widening the gap between the workpiece and the electrode tool, the concave sections whose shapes correspond to the continuous hole patterns of the masking member are formed with greater precision on the workpiece.

[0018] In one aspect of the present embodiment, a mixed solution containing a surface-active agent may be used as the electrolytic solution. As a result, various particles such as electrolytic products that elute from the workpiece are absorbed by the surface-active agent within the electrolytic solution, which ensures a smooth flow of the electrolytic solution.

[0019] In the electrolytic machining method may use an ultrasonic vibration generating device that provides ultrasonic vibration to the electrolytic solution. As a result, various particles such as electrolytic products that elute from the workpiece are made to flow smoothly due to the ultrasonic vibration provided to the electrolytic solution.

[0020] An electrolytic machining apparatus in accordance with one embodiment of the present invention may include a machining and storing section that retains an electrolytic solution and that stores at least a part in which a workpiece and an electrode tool are disposed opposite to each other and immersed in the electrolytic solution, and a workpiece supporting member that positions the machining surface of the workpiece at a depth of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section.

[0021] By the electrolytic machining apparatus having such a structure, due to the fact that the machining surface of the workpiece is positioned by the workpiece supporting member at a depth of about 5 mm or deeper from the surface of the electrolytic solution retained in the machining and storing section, there is virtually no amount of air, especially oxygen entering the electrolytic solution, which ensures a high quality electrolytic machining; at the same time, due to the fact that the machining surface of the workpiece is positioned by the workpiece supporting member at a depth of about 35 mm or less from the surface of the electrolytic solution retained in the machining and storing section, the fluidity of the electrolytic solution is maintained favorably, which allows a smooth elimination of electrolytic products after the electrolytic machining.

[0022] In one aspect of the present embodiment, the material of a shaft member or a bearing member used in a dynamic pressure bearing device that utilizes the dynamic pressure of a lubricating fluid may be used as the workpiece described above. Because dynamic pressure generating grooves are formed as concave sections in the workpiece, the dynamic pressure generating grooves are machined with especially high precision.

[0023] Further, a masking member having continuous hole patterns as through-holes that correspond to the shapes of concave sections may be adhered to the machining surface of the workpiece described above. As a result, the electrolytic solution is supplied to flow in a gap between the masking member and the electrode tool in order to allow the electrolytic solution to enter into the continuous hole patterns of the masking member and thereby allow an electrolytic machining to take place, and thus the electrolytic solution supplied to the workpiece flows only within the continuous hole patterns of the masking member that is adhered to the workpiece. Consequently, even when the fluidity of the electrolytic solution is improved by widening the gap between the workpiece and the electrode tool, the concave sections whose shapes correspond to the continuous hole patterns of the masking member are formed with greater precision on the workpiece.

[0024] In one aspect of the present embodiment, a mixed solution containing a surface-active agent may be used as the electrolytic solution described above. As a result, various particles such as electrolytic products that elute from the workpiece are absorbed by the surface-active agent within the electrolytic solution, which ensures a smooth flow of the electrolytic solution.

[0025] The electrolytic machining apparatus in accordance with one aspect of the present embodiment may be provided with an ultrasonic vibration generating device that provides ultrasonic vibration to the electrolytic solution. As a result, various particles such as electrolytic products that elute from the workpiece are made to flow smoothly due to the ultrasonic vibration provided to the electrolytic solution.

[0026] Furthermore, in the electrolytic machining apparatus, an insulating member may be provided on at least the surface part of the masking member. As a result, energization of parts other than the continuous hole patterns of the masking member is virtually completely blocked, which causes the shapes of the concave sections to be formed with even greater precision.

[0027] Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 schematically shows a front cross-sectional view of the structure of an electrolytic machining apparatus in accordance with one embodiment of the present invention.

[0029]FIG. 2 schematically shows a side cross-sectional view of the electrolytic machining apparatus shown in FIG. 1.

[0030]FIG. 3 shows a plan view of a masking member used in the electrolytic machining apparatus shown in FIGS. 1 and 2.

[0031]FIG. 4 shows an exterior view illustrating the operating condition of the electrolytic machining apparatus shown in FIGS. 1 through 3.

[0032]FIG. 5 shows a diagram of one example of energization in the electrolytic machining apparatus shown in FIGS. 1 through 4.

[0033]FIG. 6 shows a diagram of another example of energization in the electrolytic machining apparatus shown in FIGS. 1 through 4.

[0034]FIG. 7 shows a line graph showing the relationship between the immersion positioning depth in an electrolytic solution during electrolytic machining and the electrolytic machining depth.

[0035]FIG. 8 shows a longitudinal cross-sectional view of an example of the structure of a hard disk drive (HDD) motor with a dynamic pressure bearing device manufactured through the electrolytic machining according to the present invention.

[0036]FIG. 9 shows a bottom view of an example of the structure of a thrust plate used in the dynamic pressure bearing device shown in FIG. 8.

[0037]FIG. 10 shows a plan view of an example of the structure of the thrust plate used in the dynamic pressure bearing device shown in FIG. 8.

[0038]FIG. 11 shows a longitudinal cross-sectional view of the thrust plate shown in FIGS. 9 and 10.

[0039]FIG. 12 schematically shows a front cross-sectional view of the structure of an electrolytic machining apparatus in accordance with another embodiment of the present invention.

[0040]FIG. 13 schematically shows a side cross-sectional view of the electrolytic machining apparatus shown in FIG. 12.

[0041]FIG. 14 shows an enlarged cross-sectional view in part of the electrolytic machining apparatus of FIG. 13 taken along a line III-III in FIG. 15.

[0042]FIG. 15 shows a plan view of a masking member used in the electrolytic machining apparatus shown in FIGS. 12, 13 and 14.

[0043]FIG. 16 schematically shows an exterior view illustrating the operating condition of the electrolytic machining apparatus shown in FIGS. 12 through 15.

[0044]FIG. 17 schematically shows a front cross-sectional view of the structure of an electrolytic machining apparatus in accordance with yet another embodiment of the present invention.

[0045]FIG. 18 schematically shows a side cross-sectional view of the structure of the electrolytic machining apparatus shown in FIG. 17.

[0046]FIG. 19 shows a front view of a pattern structure on the end part of an electrode tool used in the electrolytic machining apparatus shown in FIGS. 17 and 18.

[0047]FIG. 20 shows a cross-sectional view taken along a line IV-IV in FIG. 19.

[0048]FIG. 21 schematically shows an exterior view illustrating the operating condition of the electrolytic machining apparatus shown in FIGS. 17 through 20.

[0049]FIG. 22 schematically shows a side view of one example of a conventional electrolytic machining apparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0050] Preferred embodiments of the present invention are described below with reference to the accompanying drawings. First, the overall structure of a hard disk drive device (HDD) to which the manufacturing methods according to the present invention are applied is described.

[0051]FIG. 8 shows an overall view of a shaft-rotation type HDD spindle motor. The HDD spindle motor may consist of a stator assembly 10, which is a fixed member, and a rotor assembly 20, which is a rotating member assembled onto the top of the stator assembly 10. The stator assembly 10 has a fixed frame 11, which is screwed to a fixed base, not shown. The fixed frame 11 is formed with an aluminum material to achieve a lighter weight; on the inner circumference surface of a ring-shaped bearing holder 12 formed upright in the generally center part of the fixed frame 11 is a bearing sleeve 13, which is a fixed bearing member formed in the shape of a hollow cylinder and joined to the bearing holder 12 through press fit or shrink fit. The bearing sleeve 13 is formed with a copper material such as phosphor bronze in order to more easily machine holes with small diameters.

[0052] A stator core 14, which consists of a laminate of electromagnetic steel plates, is mounted on the outer circumference mounting surface of the bearing holder 12. A drive coil 15 is wound on each salient pole section provided on the stator core 14.

[0053] A rotor shaft 21 that composes the rotor assembly 20 is inserted in a freely rotatable manner in a center hole provided in the bearing sleeve 13. This means that a dynamic pressure surface formed on an inner circumference wall section of the bearing sleeve 13 and a dynamic pressure surface formed on an outer circumference surface of the rotor shaft 21 are positioned opposite to each other in the radial direction and in close proximity, and radial dynamic pressure bearing sections RB are formed in a minuscule gap section between them. More specifically, the dynamic pressure surface on the bearing sleeve 13 side and the dynamic pressure surface on the rotor shaft 21 side of each of the radial dynamic pressure bearing sections RB are positioned opposite to each other in a circular fashion across a minuscule gap of several μm, and a lubricating fluid such as lubricating oil, magnetic fluid or air is filled or present in a continuous manner in the axial direction in a bearing space formed by the minuscule gap.

[0054] Herringbone-shaped radial dynamic pressure generating grooves (not shown) are provided on at least one of the dynamic pressure surfaces of the bearing sleeve 13 and the rotor shaft 21. For example, the herringbone-shaped radial dynamic pressure generating grooves may be concavely formed in a ring shape in two blocks separated in the axial direction. During rotation, a pumping effect of the radial dynamic pressure generating grooves pressurizes the lubricating fluid to generate dynamic pressure, and a rotating hub 22, which is described later, together with the rotor shaft 21 becomes shaft-supported in a non-contact manner in the radial direction with the bearing sleeve 13 due to the dynamic pressure of the lubricating fluid.

[0055] The rotating hub 22 that with the rotor shaft 21 composes the rotor assembly 20 is a generally cup-shaped member, and a joining hole 22 a provided in the center part of the rotating hub 22 is joined in a unitary fashion with the top end part of the rotor shaft 21 through press fit or shrink fit. A recording medium such as a magnetic disk is fixed to the rotating hub 22 with a clamper, not shown. In other words, the rotating hub 22 has a generally cylindrical body section 22 b, which has a recording medium disk mounted on its outer circumference section, and a ring-shaped drive magnet 22 c attached towards the bottom on the inner circumference wall surface of the body section 22 b. The ring-shaped drive magnet 22 c is positioned in a ring-shaped manner in close proximity to and opposite to the outer circumference end surface of the stator core 14.

[0056] In the meantime, a disk-shaped thrust plate 23 is fixed by a plate fixing screw 24 at the bottom end part of the rotor shaft 21, as shown in FIGS. 9, 10 and 11. The thrust plate 23 is positioned to be contained within a cylindrically-shaped depressed section 13 a (see FIG. 8), which is concavely formed in the center part of the bearing sleeve 13 towards the bottom, and a dynamic pressure surface on the top surface of the thrust plate 23 is positioned within the depressed section 13 a of the bearing sleeve 13 opposite to a dynamic pressure surface of the bearing sleeve 13 in close proximity to each other in the axial direction.

[0057] Herringbone-shaped thrust dynamic pressure generating grooves 23 a are formed on the dynamic pressure surface on the top surface of the thrust plate 23, as shown especially in FIG. 10, through an electrolytic machining method described later, and a top thrust dynamic pressure bearing section SBa is formed in the gap part between the opposing dynamic pressure surfaces of the thrust plate 23 and the bearing sleeve 13.

[0058] A counter plate 16 is positioned in close proximity to a dynamic pressure surface on the bottom surface of the thrust plate 23. The counter plate 16 may be a disk-shaped member with a relatively large diameter. The counter plate 16 is positioned to close off the opening part at the bottom of the bearing sleeve 13, and the outer circumference part of the counter plate 16 is fixed to the bearing sleeve 13.

[0059] Herringbone-shaped thrust dynamic pressure generating grooves 23 b are formed on the dynamic pressure surface on the bottom surface of the thrust plate 23 as shown especially in FIG. 9 through an electrolytic machining method described later, and a bottom thrust dynamic pressure bearing section SBb is thereby formed.

[0060] The two dynamic pressure surfaces of the thrust plate 23 and the respective opposing dynamic pressure surface of the bearing sleeve 13 and of the counter plate 16 in close proximity thus form a set of thrust dynamic pressure bearing sections SBa and SBb that are positioned adjacent to each other in the axial direction; each opposing set of dynamic pressure surfaces are positioned opposite of each other in the axial direction across a minuscule gap of several micrometers. The lubricating fluid such as oil, magnetic fluid or air is filled or present in the bearing spaces consisting of the minuscule gaps in a continuous manner in the axial direction through a pathway on the outer circumference of the thrust plate 23. During rotation, a pumping effect caused by the thrust dynamic pressure generating grooves 23 a and 23 b provided on the thrust plate 23 pressurizes the lubricating fluid to generate dynamic pressure; and the dynamic pressure of the lubricating fluid causes the rotor shaft 21 and the rotating hub 22 to be shaft-supported in the thrust direction in a floating, non-contact state.

[0061] Next, descriptions are made as to the structure of an electrolytic machining apparatus used to manufacture the thrust dynamic pressure generating grooves 23 a and 23 b on the thrust plate 23 in accordance with an embodiment of the present invention.

[0062] As indicated in FIGS. 1, 2, 3 and 4, a concave section for mounting a workpiece (i.e., a workpiece mounting concave section) is provided in the generally center part of a workpiece supporting jig 32 attached to a main body base section 31. A material (hereinafter called a thrust plate material) 23′ that it to become the thrust plate 23 is lowered and held horizontally as a workpiece in the workpiece mounting concave section. The thrust plate material 23′ may be formed from a stainless steel material according to the present embodiment.

[0063] A masking member 33 made from a thin plate insulating member is coherently mounted on the top surface of the thrust plate material 23′. The masking member 33 is a circular member whose diameter is larger than the outer diameter of the thrust plate material 23′, and the outer circumference edge part of the masking material 33 is pressed downward and fixed to the workpiece supporting jig 32 by a cap-shaped member 34.

[0064] Furthermore, as shown in FIG. 3, the masking member 33 has continuous hole patterns 33 a formed as through-holes in shapes that correspond to the thrust dynamic pressure generating grooves 23 a and 23 b. The masking member 33 may preferably have an insulating material formed at least on its surface part, and thin ceramic materials or stainless steel plates (SUS) with electrocoating or ceramic coating, or resin plates, may be used as the masking member 33. The plate thickness of the masking member 33 is approximately 0.05 mm to 0.1 mm.

[0065] In the meantime, immediately above the thrust plate material 23′ and the masking member 33 is positioned upright an electrode tool 35, which consists of a hollow rod-shaped member, in the generally vertical direction. The electrode tool 35 is fixed to or held by a main body arm section 36 that extends above the main body base section 31, and the bottom end part of the electrode tool 35 is positioned to form a gap δ of approximately 1 mm, for example, with the masking member 33 during electrolytic machining. Furthermore, a negative pole (−pole) of a DC power source with an output voltage of approximately 5V to 15V, for example, is connected to the electrode tool 35, while the positive pole (+pole) of the DC power source is connected to the thrust plate material 23′, which is the workpiece.

[0066] In the center part of the electrode tool 35, a solution pathway 35 a is formed as a through-hole in the axial direction, and an electrolytic solution is delivered by an electrolytic solution supply device (for example, a pump), not shown, from the top end of the solution pathway 35 a. The electrolytic solution used may be 10-30 wt. % NaNo₂ solution, for example. The electrolytic solution delivered from the top of the electrode tool 35 travels through the solution pathway 35 a and an exit section provided on the bottom end and falls onto the masking member 33 and the thrust plate material 23′. The electrolytic solution supplied to the center part flows radially outward in radial direction and is collected in a collection container, not shown. The electrolytic solution used may also be a 3-10 wt. % KOH, 3-10 wt. % NaOH or 5-15 wt. % Na₂Co₃.

[0067] In the meantime, the electrolytic solution flows while an appropriate amount thereof is retained within a machining and storing section 41, which is provided to cover from the outer circumference side the part where the thrust plate material 23′, which is the workpiece, and the electrode tool 35 are positioned opposite to each other. The machining and storing section 41 has a ring-shaped wall section 41 a that is formed upright on the top surface of the cap member 34 of the workpiece supporting jig 32. The electrolytic solution is retained inside the ring-shaped wall section 41 a to maintain a predetermined solution surface height, and the part where the thrust plate material 23′ and the electrode tool 35 oppose each other is positioned to be immersed in the electrolytic solution.

[0068] The machining surface, which is the top surface, of the thrust plate material 23′, which is the workpiece, held by the workpiece supporting jig 32 is positioned at a depth in the range of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section 41. The reasons for positioning the machining surface at a depth in the range of about 5 mm to about 35 mm are described later.

[0069] As the electrolytic solution is allowed to flow in the gap δ between the electrode tool 35 and the masking member 33, as well as the thrust plate material 23′, that are set in the electrolytic solution, energization takes place across the electrode tool 35 and the thrust plate material 23′. The electrolytic solution seeps into the continuous hole patterns 33 a provided in the masking member 33, so that it flows as it comes into contact with the surfaces of the thrust plate material 23′ that are exposed from the masking member 33. When the parts of the thrust plate material 23′ that are in contact with the electrolytic solution elute electrochemically, the electrolytic machining of the thrust plate material 23′ takes place.

[0070] A vibrator 37 that constitutes an ultrasonic vibration generating device is attached at the top most end part of the electrode tool 35. The vibrator 37 according to the present embodiment is a horn-type that amplifies vibration amplitude to about 20-22 μm. By vibrating the electrode tool 35, ultrasonic vibration is provided to the electrolytic solution.

[0071] In the present embodiment, energization for electrolytic machining and energization for ultrasonic vibration take place independently of each other, and the actual mode of energization is as shown in FIG. 5, where energization Pa for electrolytic machining and energization Pb for ultrasonic vibration are generated by rectangular pulse currents that alternate, or as shown in FIG. 6, where energization Ca with relatively long width for electrolytic machining and energization Cb for ultrasonic vibration partially overlap. In either method, particles such as electrolytic products can be eliminated through ultrasonic vibration while the electrolytic machining takes place.

[0072] In one aspect, a mixed solution that includes a surface-active agent is used as the electrolytic solution. The surface-active agent used in the present embodiment is an alkylether non-ionic activator, and the amount added is 0.03 vol. % or more. This addition amount is based on experiment results shown in table 1.

[0073] Table 1 shows the number of residual metal chips contained in electrolytic solution after performing an electrolytic machining for 60 seconds on a work material consisting of a stainless steel material (SUS 420) with inner diameter of 5.0 mm and thickness of 12 mm, while varying the concentration of the surface-active agent between 0% and 5%. TABLE 1 Number of Residual Metal Chips After Machining SUS 420 Material Concentration/ Machining Time 0%; 60 sec. 0.03%; 60 sec. 0.05%; 60 sec. 1%; 60 sec. 2%; 60 sec. 5%; 60 sec. Average Value 15,425 552 276 0.01 0 0 Maximum Value 24,573 891 828 2 0 0

[0074] It is understood from Table 1 that the number of residual metal chips is dramatically lowers when the volume ratio of the surface-active agent is 0.03 vol. % or more compared to when the concentration is 0 vol. %, which indicates that the surface-active agent is working effectively. Furthermore, when the volume ratio is 2 vol. % or more, the number of residual metal chips is virtually zero. On the other hand, increasing the volume ratio of the surface-active agent to 5 vol. % or more does not change the property of the machining itself; consequently, the volume ratio may preferably be set at around 2 vol. %.

[0075] According to the electrolytic machining method for a dynamic pressure bearing device using the electrolytic machining apparatus having such a structure, the electrolytic solution supplied to the thrust plate material 23′, which is the workpiece, flows only into the continuous hole patterns 33 a of the masking member 33 that is adhered to the thrust plate material 23′; consequently, even if the gap between the electrode tool 35 and the masking member 33, as well as the thrust plate material 23′, is widened to increase the fluidity of the electrolytic solution, the dynamic pressure generating grooves 23 a and 23 b having shapes that correspond to the continuous hole patterns 33 a of the masking member 33 can be formed with high precision on the thrust plate material 23′.

[0076] According to the present embodiment, the machining surface of the thrust plate material 23′, which is the workpiece, is positioned at a depth of about 5 mm or deeper from the surface of the electrolytic solution retained in the machining and storing section 41. As a result, there is virtually no amount of air, especially oxygen, that may enter the electrolytic solution, which ensures a high quality electrolytic machining. At the same time, the machining surface of the thrust plate material 23′, which is the workpiece, is positioned at a depth of about 35 mm or shallower from the surface of the electrolytic solution retained in the machining and storing section 41. As a result, the fluidity of the electrolytic solution is maintained favorably, which allows a smooth elimination of electrolytic products after the electrolytic machining.

[0077] When the relationship between the positioning depth of the machining surface of the thrust plate material 23′, which is the workpiece, from the surface of the electrolytic solution and the machining depth achieved through electrolytic machining was studied, the results were as indicated in table 2 and FIG. 7. TABLE 2 Relationship between Immersion Depth in Solution during Electrolytic Machining and Fluctuations in Electrolysis Depths Depth (mm) 0.0 1.0 2.0 5.0 9.0 15.0 35.0 55.0 Maximum 13.9 9.1 8.8 9.7 9.3 9.4 9.5 8.7 Value Median Value 11.17 7.57 7.55 7.93 8.10 8.34 8.25 7.44 Minimum 7.8 8.2 6.9 6.7 7.5 7.7 7.6 6.4 Value

[0078] As Table 2 and FIG. 7 make clear, when the machining surface of the thrust plate material (stainless steel material) 23′, which is the workpiece, is set at a position shallower than point A in FIG. 7, i.e., 5 mm, there is a tendency for smut (dissolution residue) to develop from the electrolytic solution in parts that were electrolytically processed. On the other hand, when the positioning depth of the machining surface is greater than point B in FIG. 7, i.e., 35 mm, the machining depth becomes gradually shallower as a result of voltage drop.

[0079] In contrast to these, when the positioning depth of the machining surface is set between point A (5 mm) and point B (35 mm) according to the present invention, the machining depth achieved through electrolytic machining stabilizes considerably. When the voltage value, current value and pulse width for implementing electrolytic machining were changed, the value of the machining depth did change somewhat with these changes; however, the range in which stable machining depth can be obtained remained virtually unchanged in every case, and extremely favorable results were obtained in the range of 5 mm to 35 mm under various conditions.

[0080] In the electrolytic machining according to the present embodiment, the masking member 33 is formed with the insulating member. As a result, energization of parts other than the continuous hole patterns 33 a of the masking member 33 is virtually completely blocked, which leads to the shapes of the dynamic pressure generating grooves 23 a and 23 b to be formed with even higher precision.

[0081] Further in the electrolytic machining according to the present embodiment, due to the fact that a mixed solution that includes a surface-active agent is used as the electrolytic solution, various particles such as electrolytic products from the thrust plate material 23′, which is the workpiece, are absorbed by the surface-active agent within the electrolytic solution, which ensures a smooth flow of the electrolytic solution.

[0082] In addition, in the electrolytic machining according to the present embodiment, the ultrasonic vibration generating device 37 that provides ultrasonic vibration to the electrolytic solution is provided, such that various particles such as electrolytic products that elute from the workpiece are made to flow smoothly by the ultrasonic vibration provided to the electrolytic solution.

[0083] According to the present embodiment, energization for electrolytic machining and energization for ultrasonic vibration take place independently of each other, and the energization Pa and Ca for electrolytic machining and energization Pb and Cb for ultrasonic vibration are alternated or at least partially overlapped. As a result, energization for electrolytic machining and energization for ultrasonic vibration can be switched as necessary depending on the status of the electrolytic machining, thereby ensuring the best machining condition at all times.

[0084] Next, FIGS. 12, 13, 14, 15 and 16 show another embodiment. At the center part of a masking member 33 is formed a supply opening 33 a, which allows in an electrolytic solution, as a through-hole formed in the axial direction. From the supply opening 33 a, continuous hole patterns 33 b, which include shapes that correspond to thrust dynamic pressure generating grooves 23 a and 23 b, are formed to extend radially outward in the radial direction. Each of the continuous hole patterns 33 b is formed as a continuous hole in the axial direction and is provided to extend further in the radial direction as shown in FIG. 15 outward of the outer circumference edge section (indicated by a broken line) of a part that corresponds to the thrust dynamic pressure generating groove 23 a or 23 b, which is shaded. At the extended end part outward in the radial direction of each continuous hole pattern 33 b is a discharge opening 33 c, which discharges the electrolytic solution to the outside.

[0085] Immediately above the masking member 33 is positioned upright an electrode tool 35, which consists of a hollow rod-shaped member, in the generally vertical direction and abutting the masking member 33. The electrode tool 35 is fixed to a main body arm section 36 that extends above a main body base section 31, and the bottom end part of the electrode tool 35 is positioned coherently to the top surface of the masking member 33 to press it downward in the axial direction.

[0086] The electrolytic solution in accordance with the present embodiment also flows while an appropriate amount thereof is retained within a machining and storing section 41, which is provided to cover the part where a thrust plate material 23′, which is a workpiece, and the electrode tool 35 are positioned opposite to each other. The flow (charge and discharge) of the electrolytic solution may be appropriately controlled to maintain the thrust plate material 23′ at an appropriate depth from the surface of the electrolytic solution. The machining and storing section 41 has a ring-shaped wall section 41 a that is formed upright on the top surface of a workpiece supporting jig 32; the electrolytic solution is retained inside the ring-shaped wall section 41 a to maintain a predetermined solution surface height, and the part where the thrust plate material 23′ and the electrode tool 35 oppose each other is immersed in the electrolytic solution.

[0087] The machining surface, which is the top surface, of the thrust plate material 23′, which is the workpiece, held by the workpiece supporting jig 32 is positioned at a depth in the range of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section 41.

[0088] The electrolytic solution thus provided to the center part of the masking member 33 from the electrode tool 35 flows into the continuous hole patterns 33 b provided in the masking member 33 and comes into contact with the exposed surfaces of the thrust plate material 23′ as it flows in one direction outward in the radial direction. When energization takes place across the electrode tool 35 and the thrust plate material 23′ in this state, the parts of the thrust plate material 23′ that are in contact with the electrolytic solution elute electrochemically, and the electrolytic machining of the thrust plate material 23′ takes place.

[0089] Since each continuous hole pattern 33 b of the masking member 33 is provided to extend in the radial direction to the outer side of the thrust plate material 23′, the electrolytic solution that flows within each continuous hole pattern 33 b further outward in the radial direction from the outer circumference edge of the thrust plate material 23′ is discharged outside through the discharge opening 33 c, which is provided at the outermost circumference part of each continuous hole pattern 33 b, and is collected in a collection container not shown, and re-circulated.

[0090] In the electrolytic machining of a dynamic pressure bearing device according to the present embodiment, due to the fact that the continuous hole patterns 33 b of the masking member 33 are extended in the radial direction outward of the outer circumference edge section of the thrust plate material 23′ so that the electrolytic solution can be discharged from the discharge openings 33 c provided at the outer extended end parts of the continuous hole patterns 33 b, the electrolytic solution supplied from the electrode tool 35 can flow favorably into the continuous hole patterns 33 b of the masking member 33.

[0091]FIGS. 17, 18, and 21 shows still another embodiment. In the generally center part of a workpiece supporting jig 32 attached to a main body base section 31, a workpiece mounting concave section is provided. A material (hereinafter called a thrust plate material) 23′ of a thrust plate 23 is lowered and mounted as a workpiece into the workpiece mounting concave section. Immediately above the thrust plate material 23′ is positioned upright an electrode tool 35, which consists of a rod-shaped member, in the generally vertical direction. The electrode tool 35 is held by a main body arm section 36 that extends above the main body base section 31, and the bottom end part of the electrode tool 35 is positioned to form a gap δ with the thrust plate material 23′.

[0092] Convexly formed patterns 35 a that are shaped to correspond to thrust dynamic pressure generating grooves 23 a and 23 b are provided at the bottom end surface of the electrode tool 35, as shown especially in FIGS. 19 and 20. Parts other than the patterns 35 a are filled up with an insulator 35 b made of resin, so that the end surface of the electrode tool 35 is a flat surface. The end surface including the patterns 35 a is positioned opposite the thrust plate material 23′.

[0093] In the meantime, in a gap δ between the electrode tool 35 and the thrust plate material 23′, which is the workpiece, an electrolytic solution is supplied to flow in the direction that is generally orthogonal to the axial direction. The electrolytic solution is delivered by an electrolytic solution supply device (e.g., a pump), not shown. The electrolytic solution in the present embodiment also flows while an appropriate amount thereof is retained within a machining and storing section 41, which is provided to cover the part where the thrust plate material 23′, which is the workpiece, and the electrode tool 35 are positioned opposite to each other. The machining and storing section 41 has a ring-shaped wall section 41 a that is formed upright on the top surface of a workpiece supporting jig 32; the electrolytic solution is retained inside the ring-shaped wall section 41 a to maintain a predetermined solution surface height, and the part where the thrust plate material 23′ and the electrode tool 35 oppose each other is immersed in the electrolytic solution.

[0094] The machining surface, which is the top surface, of the thrust plate material 23′, which is the workpiece, held by the workpiece supporting jig 32 is positioned at a depth in the range of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section 41.

[0095] According to the present embodiment having such a structure, even when the electrode tool 35 is placed in close proximity with the thrust plate material 23′, which is the workpiece, ultrasonic vibration provided to the electrolytic solution causes various particles such as electrolytic products that elute from the thrust plate material 23′ to flow smoothly, thereby maintaining a favorable electrolytic machining; as a result, the shapes of the patterns 35 a provided on the electrode tool 35 is formed with high precision on the thrust plate material 23′.

[0096] The above describes in detail the preferred embodiments of the present invention, but many modifications can be made without departing from the subject matter of the present invention.

[0097] For example, stainless steel material is used as the workpiece (i.e., the thrust plate material 23′) in the embodiments described above, but the present invention can be similarly applied to copper metals such as phosphor bronze.

[0098] Furthermore, although the embodiments are applications of the present invention to a dynamic pressure bearing device of a hard disk drive (HDD) motor, the present invention can be similarly applied to other types of dynamic pressure bearing devices, as well as to electrolytic machining methods for various types of workpieces.

[0099] As described above, in an electrolytic machining method or an electrolytic machining apparatus in accordance with embodiments of the present invention, the machining surface of a workpiece is positioned at a depth of about 5 mm to about 35 mm from the surface of an electrolytic solution retained in a machining and storing section to virtually eliminate any amount of air, especially oxygen that may enter, from the electrolytic solution, and to favorably maintain the fluidity of the electrolytic solution in order to smoothly eliminate electrolytic products after electrolytic machining; consequently, high quality and extremely high precision electrolytic machining can be achieved easily.

[0100] Furthermore, the material of a shaft member or a bearing member used in a dynamic pressure bearing device that utilizes the dynamic pressure of a lubricating fluid may be used as the workpiece, and dynamic pressure generating grooves are formed as concave sections in the workpiece in order to achieve high precision in the machining especially of the dynamic pressure generating grooves; consequently, dynamic pressure bearing devices can be manufactured in high quality and with extremely high precision.

[0101] Moreover, a masking member having as through-holes continuous hole patterns that correspond to the shapes of concave sections is adhered to the machining surface of the workpiece. As a result, the electrolytic solution is supplied to flow in a gap between the masking member and the electrode tool in order to allow the electrolytic solution to enter into the continuous hole patterns of the masking member and thereby allow an electrolytic machining to take place; and the electrolytic solution is allowed to flow only within the continuous hole patterns of the masking member that is adhered to the workpiece in order to easily and with high precision form on the workpiece the concave sections whose shapes correspond to the continuous hole patterns; consequently, high precision electrolytic machining can be performed inexpensively using a simple structure, which can significantly improve the practicality of electrolytic machining.

[0102] A mixed solution containing a surface-active agent may be used as the electrolytic solution in order to ensure a smooth flow of the electrolytic solution. Consequently, effects described above can be further enhanced.

[0103] Also, in accordance with the present invention, an ultrasonic vibration generating device that provides ultrasonic vibration to the electrolytic solution is provided in order to allow the electrolytic solution, which contains various particles such as electrolytic products that elute from the workpiece, to flow smoothly due to the ultrasonic vibration; consequently, effects described above can be further enhanced.

[0104] Furthermore, an insulating member is provided on at least the surface part of the masking member to virtually completely block energization of parts other than the continuous hole patterns of the masking member in order to form the shapes of the concave sections with even greater precision; consequently, effects described above can be further enhanced.

[0105] While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.

[0106] The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 

What is claimed is:
 1. An electrolytic machining method for electrolytically machining a workpiece that is positioned opposite to an electrode tool with an electrolytic solution filled between the electrode tool and the workpiece, the method comprising the steps of: immersing at least a part of an opposing section of the workpiece and the electrode tool in an electrolytic solution reserved in a machining and storing section; positioning the opposing section of the workpiece and the electrode tool at a predetermined depth from a surface of the electrolytic solution; and supplying the electrolytic solution to be filled in a gap at the opposing section of the workpiece and the electrode tool and electrolytically machining a machining surface of the workpiece.
 2. An electrolytic machining method according to claim 1, wherein the machining surface of the workpiece is positioned at a depth of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section.
 3. An electrolytic machining method according to claim 1, wherein the opposing section of the workpiece and the electrode tool is completely immersed in the electrolytic solution retained in the machining and storing section.
 4. An electrolytic machining method according to claim 3, wherein the machining surface of the workpiece is positioned at a depth of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section.
 5. An electrolytic machining method according to claim 1, wherein the opposing section of the workpiece and the electrode tool includes an end face section of the electrode tool that faces the machining surface of the workpiece and that is immersed in the electrolytic solution reserved in the machining and storing section.
 6. An electrolytic machining method according to claim 1, wherein the workpiece is composed of a material of a shaft member used in a dynamic pressure bearing device that utilizes a dynamic pressure of a lubricating fluid, and dynamic pressure generating grooves are formed as concave sections in the workpiece.
 7. An electrolytic machining method according to claim 1, wherein the workpiece is composed of a material of a bearing member used in a dynamic pressure bearing device that utilizes a dynamic pressure of a lubricating fluid, and dynamic pressure generating grooves are formed as concave sections in the workpiece.
 8. An electrolytic machining method according to claim 1, further comprising the step of adhering a masking member that has continuous hole patterns formed as through-holes that correspond to the shapes of the concave sections to the machining surface of the workpiece.
 9. An electrolytic machining method according to claim 8, wherein the electrolytic solution is supplied to flow in a gap between the masking member and the electrode tool to allow the electrolytic solution to enter the continuous hole patterns of the masking member and an electric current is applied across the workpiece and the electrode tool to thereby allow an electrolytic machining to take place.
 10. An electrolytic machining method according to claim 1, wherein the electrolytic solution is a mixed solution containing a surface-active agent.
 11. An electrolytic machining method according to claim 1, further comprising the step of applying ultrasonic vibration to the electrolytic solution.
 12. An electrolytic machining apparatus comprising: an electrode tool; a machining and storing section that reserves an electrolytic solution; and a supporting member that retains at least a part of the electrode tool immersed in the electrolytic solution and positions a machining surface of a workpiece at a predetermined depth from a surface of the electrolytic solution stored in the machining and storing section.
 13. An electrolytic machining apparatus according to claim 12, wherein the supporting member positions the machining surface of a workpiece at a depth of about 5 mm to about 35 mm from the surface of the electrolytic solution retained in the machining and storing section.
 14. An electrolytic machining apparatus according to claim 12, further comprising a flow control device that controls charging and discharging of the electrolytic solution in and out of the machining and storing section to maintain the surface level of the electrolytic solution in the machining and storing section at a specified level.
 15. An electrolytic machining apparatus according to claim 13, wherein the workpiece is composed of a material of a shaft member used in a dynamic pressure bearing device that utilizes a dynamic pressure of a lubricating fluid.
 16. An electrolytic machining apparatus according to claim 13, wherein the workpiece is composed of a material of a bearing member used in a dynamic pressure bearing device that utilizes a dynamic pressure of a lubricating fluid.
 17. An electrolytic machining apparatus according to claim 12, wherein a masking member having continuous through-hole patterns that correspond to concave sections to be formed on the machining surface of the workpiece is adhered to the machining surface of the workpiece.
 18. An electrolytic machining apparatus according to claim 12, wherein the electrolytic solution is a mixed solution containing a surface-active agent.
 19. An electrolytic machining apparatus according to claim 12, further comprising an ultrasonic vibration generating device that provides ultrasonic vibration to the electrolytic solution.
 20. An electrolytic machining apparatus according to claim 17, wherein an insulating member is provided on at least the surface part of the masking member to substantially block energization of parts other than the continuous hole patterns of the masking member. 